Effects of peroxisome proliferator-activated receptor alpha/delta agonists on HDL-cholesterol in vervet monkeys

Exploring the Regulation of Jiangtang Tiaozhi Formula on the Biological Network of Obese T2DM Complicated With Dyslipidemia Based on Clinical Transcriptomics

OBJECTIVE

To use systems biology to explore the biomolecular network mechanism of the Jiangtang Tiaozhi Recipe (JTTZR) in the intervention of obese Type 2 diabetes (T2DM) patients with dyslipidemia.

METHODS

Twelve patients with obese type 2 diabetes mellitus and dyslipidemia (traditional Chinese medicine syndrome differentiation was excess heat syndrome of the stomach and intestines) were treated with JTTZR for 24 weeks, and 12 patients were included in the healthy control group. First, blood samples from 6 patients in each group (disease group before treatment, disease group after treatment, and healthy control group) were collected for RNA microarray analysis. Quantitative polymerase chain reaction (qPCR) was used to validate these target lncRNAs and mRNAs. Finally, a detailed analysis of the differences in the disease group before treatment vs. the healthy control group and the disease group after treatment vs. the disease group before treatment was undertaken. In addition, we focused on disease-related pathways and analyzed the correlation between the differential expression of target lncRNAs and clinical indicators.

RESULTS

(1) Disease group before treatment vs. healthy control group: There were 557 up-regulated lncRNAs, 273 down-regulated lncRNAs, 491 up-regulated mRNAs, and 1639 down-regulated mRNAs. GO analysis and pathway analysis showed that T2DM may be related to cell proliferation in the forebrain, post-embryonic organ development, calcium signaling pathway. qPCR validation showed that the expression of XLOC-005590 and HNF1A-AS1 as target lncRNAs increased, and this was verified by gene chip analysis. (2) Disease group after treatment vs. disease group before treatment: 128 lncRNAs were upregulated, 32 lncRNAs were downregulated, 45 mRNAs were upregulated, and 140 mRNAs were downregulated. GO analysis and pathway analysis showed that JTTZR may treat T2DM through endosome transport, the insulin signaling pathway, and glycine, serine, and threonine metabolism. qPCR validation showed that in the healthy control group, XLOC_005590 was upregulated, whereas the downstream gene (ECI2) was downregulated in the disease group before treatment. However, after 24 weeks of intervention with JTTZR, XLOC_005590 was downregulated and ECI2 was upregulated compared with the disease group before treatment (0 weeks) (P <0.05).

CONCLUSION

JTTZR may interfere in patients with obese T2DM with dyslipidemia by regulating pathways such as fatty acid degradation, glycolysis/gluconeogenesis, and pyruvate metabolism.

High-intensity interval training has a greater effect on reverse cholesterol transport elements compared with moderate-intensity continuous training in obese male rats

OBJECTIVES

The present study compares the effect of high-intensity interval training (HIIT; 18 min) and moderate-intensity continuous training (MIT; 1 h) on reverse cholesterol transport (RCT) elements in obese subjects.

METHODS

Thirty adult male rats were induced high-fat diet (HFD) for 12 weeks. After four weeks, the rats were randomly divided into three groups while simultaneously continuing the HFD for the remaining eight weeks. Group specificities were HFD-control, HFD-MIT and HFD-HIIT. The rats were sacrificed 48 h after the last training session and the samples were collected. Analysis of variance and Pearson's correlation test were used for the statistical analyses (significance level: p ≤ 0.05).

RESULTS

The results showed that both HIIT and MIT improved heart ABCA1, ABCG1, ABCG4, ABCG5, ABCG8, LXR-α and PPARγ gene expression as well as plasma Apo A1, LCAT, lipids and lipoproteins (p ≤ 0.05). Moreover, higher cardiac ABCA1, ABCG1, ABCG4, ABCG5, ABCG8 and PPARγ expression and plasma high-density lipoprotein cholesterol (p ≤ 0.05) concentrations were found in the HFD-HIIT group compared with the HFD-MIT group.

CONCLUSION

HIIT may have more cardioprotective effects than MIT against atherosclerosis, along with saving time, as supported by the changes observed in the main factors involved in the RCT process.

Nanopharmaceutical-based regenerative medicine: a promising therapeutic strategy for spinal cord injury

Spinal cord injury (SCI) is a neurological disorder that can lead to loss of perceptive and athletic function due to the severe nerve damage. To date, pieces of evidence detailing the precise pathological mechanisms in SCI are still unclear. Therefore, drug therapy cannot effectively alleviate the SCI symptoms and faces the limitations of systemic administration with large side effects. Thus, the development of SCI treatment strategies is urgent and valuable. Due to the application of nanotechnology in pharmaceutical research, nanopharmaceutical-based regenerative medicine will bring colossal development space for clinical medicine. These nanopharmaceuticals (i.e. nanocrystalline drugs and nanocarrier drugs) are designed using different types of materials or bioactive molecules, so as to improve the therapeutic effects, reduce side effects, and subtly deliver drugs, etc. Currently, an increasing number of nanopharmaceutical products have been approved by drug regulatory agencies, which has also prompted more researchers to focus on the potential treatment strategies of SCI. Therefore, the purpose of this review is to summarize and elaborate the research progress as well as the challenges and future of nanopharmaceuticals in the treatment of SCI, aiming to promote further research of nanopharmaceuticals in SCI.

Sirtuins-Mediated System-Level Regulation of Mammalian Tissues at the Interface between Metabolism and Cell Cycle: A Systematic Review

Sirtuins are a family of highly conserved NAD+-dependent proteins and this dependency links Sirtuins directly to metabolism. Sirtuins' activity has been shown to extend the lifespan of several organisms and mainly through the post-translational modification of their many target proteins, with deacetylation being the most common modification. The seven mammalian Sirtuins, SIRT1 through SIRT7, have been implicated in regulating physiological responses to metabolism and stress by acting as nutrient sensors, linking environmental and nutrient signals to mammalian metabolic homeostasis. Furthermore, mammalian Sirtuins have been implicated in playing major roles in mammalian pathophysiological conditions such as inflammation, obesity and cancer. Mammalian Sirtuins are expressed heterogeneously among different organs and tissues, and the same holds true for their substrates. Thus, the function of mammalian Sirtuins together with their substrates is expected to vary among tissues. Any therapy depending on Sirtuins could therefore have different local as well as systemic effects. Here, an introduction to processes relevant for the actions of Sirtuins, such as metabolism and cell cycle, will be followed by reasoning on the system-level function of Sirtuins and their substrates in different mammalian tissues. Their involvement in the healthy metabolism and metabolic disorders will be reviewed and critically discussed.

The Mystery behind the Pineal Gland: Melatonin Affects the Metabolism of Cholesterol

Melatonin may be considered a cardioprotective agent. Since atherogenesis is partly associated with the metabolism of lipoproteins, it seems plausible that melatonin affects cardiovascular risk by modulating the metabolism of cholesterol and its subfractions. Moreover, cholesterol-driven atherogenesis can be hypothetically reduced by melatonin, mainly due to the minimalization of harmful reactions triggered in the cardiovascular system by the reactive oxygen species-induced toxic derivatives of cholesterol. In this review, we attempted to summarize the available data on the hypolipemizing effects of melatonin, with some emphasis on the molecular mechanisms underlying these reactions. We aimed to attract readers' attention to the numerous gaps of knowledge present in the reviewed field and the essential irrelevance between the findings originating from different sources: clinical observations and in vitro mechanistic and molecular studies, as well as preclinical experiments involving animal models. Overall, such inconsistencies make it currently impossible to give a reliable opinion on the action of melatonin on the metabolism of lipoproteins.

Aspalathin-Rich Green Rooibos Extract Lowers LDL-Cholesterol and Oxidative Status in High-Fat Diet-Induced Diabetic Vervet Monkeys

Type 2 diabetic patients possess a two to four-fold-increased risk for cardiovascular diseases (CVD). Hyperglycemia, oxidative stress associated with endothelial dysfunction and dyslipidemia are regarded as pro-atherogenic mechanisms of CVD. In this study, high-fat diet-induced diabetic and non-diabetic vervet monkeys were treated with 90 mg/kg of aspalathin-rich green rooibos extract (Afriplex GRT) for 28 days, followed by a 1-month wash-out period. Supplementation showed improvements in both the intravenous glucose tolerance test (IVGTT) glycemic area under curve (AUC) and total cholesterol (due to a decrease of the low-density lipoprotein [LDL]) values in diabetics, while non-diabetic monkeys benefited from an increase in high-density lipoprotein (HDL) levels. No variation of plasma coenzyme Q10 (CoQ₁₀) were found, suggesting that the LDL-lowering effect of Afriplex GRT could be related to its ability to modulate the mevalonate pathway differently from statins. Concerning the plasma oxidative status, a decrease in percentage of oxidized CoQ₁₀ and circulating oxidized LDL (ox-LDL) levels after supplementation was observed in diabetics. Finally, the direct correlation between the amount of oxidized LDL and total LDL concentration, and the inverse correlation between ox-LDL and plasma CoQ₁₀ levels, detected in the diabetic monkeys highlighted the potential cardiovascular protective role of green rooibos extract. Taken together, these findings suggest that Afriplex GRT could counteract hyperglycemia, oxidative stress and dyslipidemia, thereby lowering fundamental cardiovascular risk factors associated with diabetes.

PPARs: regulators of metabolism and as therapeutic targets in cardiovascular disease. Part II: PPAR-β/δ and PPAR-γ

The PPARs are a subfamily of three ligand-inducible transcription factors, which belong to the superfamily of nuclear hormone receptors. In mammals, the PPAR subfamily consists of three members: PPAR-α, PPAR-β/δ and PPAR-γ. PPARs control the expression of a large number of genes involved in metabolic homeostasis, lipid, glucose and energy metabolism, adipogenesis and inflammation. PPARs regulate a large number of metabolic pathways that are implicated in the pathogenesis of metabolic diseases such as metabolic syndrome, Type 2 diabetes mellitus, nonalcoholic fatty liver disease and cardiovascular disease. The aim of this review is to provide up-to-date information about the biochemical and metabolic actions of PPAR-β/δ and PPAR-γ, the therapeutic potential of their agonists currently under clinical development and the cardiovascular disease outcome of clinical trials of PPAR-γ agonists, pioglitazone and rosiglitazone.

Antihypertensive effects of peroxisome proliferator-activated receptor-β/δ activation

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily of ligand-activated transcription factors, which is composed of three members encoded by distinct genes: PPARα, PPARβ/δ, and PPARγ. The biological actions of PPARα and PPARγ and their potential as a cardiovascular therapeutic target have been extensively reviewed, whereas the biological actions of PPARβ/δ and its effectiveness as a therapeutic target in the treatment of hypertension remain less investigated. Preclinical studies suggest that pharmacological PPARβ/δ activation induces antihypertensive effects in direct [spontaneously hypertensive rat (SHR), ANG II, and DOCA-salt] and indirect (dyslipemic and gestational) models of hypertension, associated with end-organ damage protection. This review summarizes mechanistic insights into the antihypertensive effects of PPARβ/δ activators, including molecular and functional mechanisms. Pharmacological PPARβ/δ activation induces genomic actions including the increase of regulators of G protein-coupled signaling (RGS), acute nongenomic vasodilator effects, as well as the ability to improve the endothelial dysfunction, reduce vascular inflammation, vasoconstrictor responses, and sympathetic outflow from central nervous system. Evidence from clinical trials is also examined. These preclinical and clinical outcomes of PPARβ/δ ligands may provide a basis for the development of therapies in combating hypertension.

Hormetic and regulatory effects of lipid peroxidation mediators in pancreatic beta cells

Nutrient sensing mechanisms of carbohydrates, amino acids and lipids operate distinct pathways that are essential for the adaptation to varying metabolic conditions. The role of nutrient-induced biosynthesis of hormones is paramount for attaining metabolic homeostasis in the organism. Nutrient overload attenuate key metabolic cellular functions and interfere with hormonal-regulated inter- and intra-organ communication, which may ultimately lead to metabolic derangements. Hyperglycemia and high levels of saturated free fatty acids induce excessive production of oxygen free radicals in tissues and cells. This phenomenon, which is accentuated in both type-1 and type-2 diabetic patients, has been associated with the development of impaired glucose tolerance and the etiology of peripheral complications. However, low levels of the same free radicals also induce hormetic responses that protect cells against deleterious effects of the same radicals. Of interest is the role of hydroxyl radicals in initiating peroxidation of polyunsaturated fatty acids (PUFA) and generation of α,β-unsaturated reactive 4-hydroxyalkenals that avidly form covalent adducts with nucleophilic moieties in proteins, phospholipids and nucleic acids. Numerous studies have linked the lipid peroxidation product 4-hydroxy-2E-nonenal (4-HNE) to different pathological and cytotoxic processes. Similarly, two other members of the family, 4-hydroxyl-2E-hexenal (4-HHE) and 4-hydroxy-2E,6Z-dodecadienal (4-HDDE), have also been identified as potential cytotoxic agents. It has been suggested that 4-HNE-induced modifications in macromolecules in cells may alter their cellular functions and modify signaling properties. Yet, it has also been acknowledged that these bioactive aldehydes also function as signaling molecules that directly modify cell functions in a hormetic fashion to enable cells adapt to various stressful stimuli. Recent studies have shown that 4-HNE and 4-HDDE, which activate peroxisome proliferator-activated receptor δ (PPARδ) in vascular endothelial cells and insulin secreting beta cells, promote such adaptive responses to ameliorate detrimental effects of high glucose and diabetes-like conditions. In addition, due to the electrophilic nature of these reactive aldehydes they form covalent adducts with electronegative moieties in proteins, phosphatidylethanolamine and nucleotides. Normally these non-enzymatic modifications are maintained below the cytotoxic range due to efficient cellular neutralization processes of 4-hydroxyalkenals. The major neutralizing enzymes include fatty aldehyde dehydrogenase (FALDH), aldose reductase (AR) and alcohol dehydrogenase (ADH), which transform the aldehyde to the corresponding carboxylic acid or alcohols, respectively, or by biding to the thiol group in glutathione (GSH) by the action of glutathione-S-transferase (GST). This review describes the hormetic and cytotoxic roles of oxygen free radicals and 4-hydroxyalkenals in beta cells exposed to nutritional challenges and the cellular mechanisms they employ to maintain their level at functional range below the cytotoxic threshold.

Activation of PPARδ: from computer modelling to biological effects

PPARδ is a ligand-activated receptor that dimerizes with another nuclear receptor of the retinoic acid receptor family. The dimers interact with other co-activator proteins and form active complexes that bind to PPAR response elements and promote transcription of genes involved in lipid metabolism. It appears that various natural fatty acids and their metabolites serve as endogenous activators of PPARδ; however, there is no consensus in the literature on the nature of the prime activators of the receptor. In vitro and cell-based assays of PPARδ activation by fatty acids and their derivatives often produce conflicting results. The search for synthetic and selective PPARδ agonists, which may be pharmacologically useful, is intense. Current rational modelling used to obtain such compounds relies mostly on crystal structures of synthetic PPARδ ligands with the recombinant ligand binding domain (LBD) of the receptor. Here, we introduce an original computational prediction model for ligand binding to PPARδ LBD. The model was built based on EC50 data of 16 ligands with available crystal structures and validated by calculating binding probabilities of 82 different natural and synthetic compounds from the literature. These compounds were independently tested in cell-free and cell-based assays for their capacity to bind or activate PPARδ, leading to prediction accuracy of between 70% and 93% (depending on ligand type). This new computational tool could therefore be used in the search for natural and synthetic agonists of the receptor.

Decreased global DNA methylation in the white blood cells of high fat diet fed vervet monkeys (Chlorocebus aethiops)

Epigenetic mechanisms are associated with the development of many chronic diseases and due to their reversible nature offer a unique window of opportunity to reverse the disease phenotype. This study investigated whether global DNA methylation correlates with dysglycemia in the vervet monkey (Chlorocebus aethiops). Diet-induced changes in DNA methylation were observed where global DNA methylation was twofold lower in monkeys fed a high fat diet (n = 10) compared to monkeys fed a standard diet (n = 15). An inverse correlation was observed between DNA methylation, blood glucose concentrations, bodyweight, and age, although the association was not statistically significant. Consumption of a high fat diet is associated with the development of metabolic disease; thus, these results suggest the use of global DNA methylation as a biomarker to assess the risk for metabolic disease. Moreover, this study provides further support for the use of the vervet monkey as a model system to study metabolic diseases such as type 2 diabetes. Integration of altered DNA methylation profiles into predictive models could facilitate risk stratification and enable intervention strategies to inhibit disease progression. Such interventions could include lifestyle modifications, for example, the increased consumption of functional foods with the capacity to modulate DNA methylation, thus potentially reversing the disease phenotype and preventing disease.

Effects of short-term niacin treatment on plasma lipoprotein concentrations in African green monkeys (Chlorocebus aethiops)

Niacin is the most effective drug available for raising levels of high-density lipoprotein (HDL) cholesterol. To evaluate its effects on plasma lipid concentrations, the authors administered a low dose of niacin to healthy, adult, female African green monkeys for 3 months. In the treated monkeys, low-density lipoprotein cholesterol concentrations decreased by 43% from baseline, whereas concentrations of HDL cholesterol and apolipoprotein A-I increased by 49% and 34%, respectively. The results suggest that in this primate model, a low dose of niacin can effectively increase concentrations of HDL cholesterol.

Peroxisome proliferator-activated receptor delta and cardiovascular disease

Recent reports have shown that peroxisome proliferator-activated receptor delta (PPARD) plays an important role in different vascular processes suggesting that PPARD is a significant modulator of cardiovascular disease. This review will focus on PPARD in relation to cardiovascular risk factors based on cell, animal and human data. Mouse studies suggest that Ppard is an important metabolic modulator that may have implications for cardiovascular disease (CVD). Specific human PPARD gene variants show no clear association with CVD but interactions between variants and lifestyle factors might influence disease risk. During recent years, development of specific and potent PPARD agonists has also made it possible to study the effects of PPARD activation in humans. PPARD agonists seem to exert beneficial effects on dyslipidemia and insulin-resistant syndromes but safety issues have been raised due to the role that PPARD plays in cell proliferation. Thus, large long term outcome as well as detailed safety and tolerability studies are needed to evaluate whether PPARD agonists could be used to treat CVD in humans.

Beneficial effects of flaxseed oil and fish oil diet are through modulation of different hepatic genes involved in lipid metabolism in streptozotocin-nicotinamide induced diabetic rats

Dietary omega-3 fatty acids have been demonstrated to have positive physiological effects on lipid metabolism, cardiovascular system and insulin resistance. Type-2 diabetes (T2DM) is known for perturbations in fatty acid metabolism leading to dyslipidemia. Our objective was to investigate beneficial effects of dietary flaxseed oil and fish oil in streptozotocin-nicotinamide induced diabetic rats. Thirty-six adult, male, Wistar rats were divided into six groups: three diabetic and three non-diabetic. Diabetes was induced by an injection of nicotinamide (110 mg/kg) and STZ (65 mg/kg). The animals received either control, flaxseed oil or fish oil (10 % w/w) enriched diets for 35 days. Both diets lowered serum triglycerides and very low-density lipoprotein cholesterol levels and elevated serum high-density lipoprotein cholesterol levels in diabetic rats, while serum total cholesterol and LDL-C levels remained unaffected. Both the diets increased omega-3 levels in plasma and RBCs of diabetic rats. Flaxseed oil diet significantly up-regulated the key transcription factor peroxisome proliferator-activated receptor-α (PPAR-α ) and down-regulated sterol regulatory element-binding protein-1 (SREBP-1) in diabetic rats, which would have increased β-oxidation of fatty acids and concomitantly reduced lipogenesis respectively, thereby reducing TG levels. Fish oil diet, on the contrary lowered serum TG levels without altering PPAR-α while it showed a non-significant reduction in SREBP-1 expression in diabetic rats. Another key finding of the study is the activation of D5 and D6 desaturases in diabetic rats by flaxseed oil diet or fish oil diets, which may have resulted in an improved omega-3 status and comparable effects shown by both diets. The reduced expression of Liver-fatty acid binding protein in diabetic rats was restored by fish oil alone, while both diets showed equal effects on adipocyte fatty acid-binding protein expression. We also observed down-regulation of atherogenic cytokines tumor necrosis factor-α and interleukin-6 by both the diets. In conclusion, dietary flaxseed oil and fish oil have therapeutic potential in preventing lipid abnormalities in T2DM.

Nonhuman primates and other animal models in diabetes research

Animal models are important for determining the pathogenesis of and potential treatments for obesity and diabetes. Nonhuman primates (NHPs) are particularly useful for studying these disorders. As in humans, type 2 diabetes mellitus is the most common form of diabetes in NHPs and occurs more often in older obese animals, with a metabolic progression from insulin resistance (IR) and impaired glucose tolerance to overt diabetes. Histopathologic changes in pancreatic islets are also similar to those seen in humans with diabetes. Initially, there is islet hyperplasia with abundant insulin production to compensate for IR, followed by insufficient insulin production with replacement of islets with islet-associated amyloid. Diabetic NHPs also have adverse changes in plasma lipid and lipoprotein concentrations, biomarkers of obesity, inflammation, and oxidative stress, and protein glycation that contribute to the numerous complications of the disease. Furthermore, sex hormones, pregnancy, and environmental factors (e.g., diet and stress) affect IR and can also contribute to diabetes progression in NHPs. Additionally, due to their similar clinical and pathologic characteristics, NHPs have been used in many pharmacological studies to assess new therapeutic agents. For these reasons, NHPs are particularly valuable animal models of obesity and diabetes for studying disease pathogenesis, risk factors, comorbidities, and therapeutic interventions.

Generation and characterization of a humanized PPARδ mouse model

BACKGROUND AND PURPOSE

Humanized mice for the nuclear receptor peroxisome proliferator-activated receptor δ (PPARδ), termed PPARδ knock-in (PPARδ KI) mice, were generated for the investigation of functional differences between mouse and human PPARδ and as tools for early drug efficacy assessment.

EXPERIMENTAL APPROACH

Human PPARδ function in lipid metabolism was assessed at baseline, after fasting or when challenged with the GW0742 compound in mice fed a chow diet or high-fat diet (HFD).

KEY RESULTS

Analysis of PPARδ mRNA levels revealed a hypomorph expression of human PPARδ in liver, macrophages, small intestine and heart, but not in soleus and quadriceps muscles, white adipose tissue and skin. PPARδ KI mice displayed a small decrease of high-density lipoprotein-cholesterol whereas other lipid parameters were unaltered. Plasma metabolic parameters were similar in wild-type and PPARδ KI mice when fed chow or HFD, and following physiological (fasting) and pharmacological (GW0742 compound) activation of PPARδ. Gene expression profiling in liver, soleus muscle and macrophages showed similar gene patterns regulated by mouse and human PPARδ. The anti-inflammatory potential of human PPARδ was also similar to mouse PPARδ in liver and isolated macrophages.

CONCLUSIONS AND IMPLICATIONS

These data indicate that human PPARδ can compensate for mouse PPARδ in the regulation of lipid metabolism and inflammation. Overall, this novel PPARδ KI mouse model shows full responsiveness to pharmacological challenge and represents a useful tool for the preclinical assessment of PPARδ activators with species-specific activity.

Effects of the PPAR-δ agonist MBX-8025 on atherogenic dyslipidemia

OBJECTIVE

Determine the effects of treatment with a selective PPAR-δ agonist±statin on plasma lipoprotein subfractions in dyslipidemic individuals.

METHODS

Ion mobility analysis was used to measure plasma concentrations of subfractions of the full spectrum of lipoprotein particles in 166 overweight or obese dyslipidemic individuals treated with the PPAR-δ agonist MBX-8025 (50 and 100 mg/d)±atorvastatin (20 mg/d) in an 8-week randomized parallel arm double blind placebo controlled trial.

RESULTS

MBX-8025 at both doses resulted in reductions of small plus very small LDL particles and increased levels of large LDL, with a concomitant reduction in large VLDL, and an increase in LDL peak diameter. This translated to reversal of the small dense LDL phenotype (LDL pattern B) in ∼90% of the participants. Modest increases in HDL particles were confined to the smaller HDL fractions. Atorvastatin monotherapy resulted in reductions in particles across the VLDL-IDL-LDL spectrum, with a significantly smaller reduction in small and very small LDL vs. MBX-8025 100 mg/d (-24.5±5.3% vs. -47.8±4.9%), and, in combination with MBX-8025, a reversal of the increase in large LDL.

CONCLUSION

PPAR-δ and statin therapies have complementary effects in improving lipoprotein subfractions associated with atherogenic dyslipidemia.

Lipid-sensing nuclear receptors in the pathophysiology and treatment of the metabolic syndrome

Metabolic syndrome (MS) is a cluster of different diseases, namely central obesity, hypertension, hyperglycemia, and dyslipidemia, together with a pro-thrombotic and pro-inflammatory state. These metabolic abnormalities are often associated with an increased risk for cardiovascular disease (CVD) and cancer. Dietary and lifestyle modifications are currently believed more effective than pharmacological therapies in the management of MS patients. Nevertheless, the relatively low grade of compliance of patients to these recommendations, as well as the failure of current therapies, highlights the need for the discovery of new pharmacological and nutraceutic approaches. A deeper knowledge of the patho-physiological events that initiate and support the MS is mandatory. Lipid-sensing nuclear receptors (NRs) are the master transcriptional regulators of lipid and carbohydrate metabolism and inflammatory responses, thus standing as suitable targets. This review focuses on the physiological relevance of the NRs (peroxisome proliferator-activated receptors, liver X receptors, and farnesoid X receptor) in the control of whole-body homeostasis, with a special emphasis on lipid and glucose metabolism, and on the relationships between metabolic unbalances, systemic inflammation, and the onset of CVD. Future perspectives and possible clinical applications are also presented.

Peroxisome Proliferators-Activated Receptor (PPAR) Modulators and Metabolic Disorders

Overweight and obesity lead to an increased risk for metabolic disorders such as impaired glucose regulation/insulin resistance, dyslipidemia, and hypertension. Several molecular drug targets with potential to prevent or treat metabolic disorders have been revealed. Interestingly, the activation of peroxisome proliferator-activated receptor (PPAR), which belongs to the nuclear receptor superfamily, has many beneficial clinical effects. PPAR directly modulates gene expression by binding to a specific ligand. All PPAR subtypes (alpha, gamma, and sigma) are involved in glucose metabolism, lipid metabolism, and energy balance. PPAR agonists play an important role in therapeutic aspects of metabolic disorders. However, undesired effects of the existing PPAR agonists have been reported. A great deal of recent research has focused on the discovery of new PPAR modulators with more beneficial effects and more safety without producing undesired side effects. Herein, we briefly review the roles of PPAR in metabolic disorders, the effects of PPAR modulators in metabolic disorders, and the technologies with which to discover new PPAR modulators.

Effects of aleglitazar, a balanced dual peroxisome proliferator-activated receptor α/γ agonist on glycemic and lipid parameters in a primate model of the metabolic syndrome

BACKGROUND

Glycemic control and management of dyslipidemia to reduce cardiovascular risk are major therapeutic goals in individuals with type 2 diabetes mellitus (T2DM). This study was performed to evaluate the effects of aleglitazar, a balanced dual peroxisome proliferator-activated receptor α/γ (PPARα/γ) agonist, on both lipid and glycemic parameters in obese, hypertriglyceridemic, insulin-resistant rhesus monkeys.

METHODS

A 135-day efficacy study was performed in six rhesus monkeys. After a 28-day baseline assessment (vehicle only), monkeys received oral aleglitazar 0.03 mg/kg per day for 42 days, followed by a 63-day washout period. Plasma levels of markers of glycemic and lipid regulation were measured at baseline, at the end of the dosing period, and at the end of the washout period.

RESULTS

Compared with baseline values, aleglitazar 0.03 mg/kg per day reduced triglyceride levels by an average of 89% (328 to 36 mg/dL; P = 0.0035 when normalized for baseline levels) and increased high-density lipoprotein cholesterol levels by 125% (46 to 102 mg/dL; P = 0.0007). Furthermore, aleglitazar reduced low-density lipoprotein cholesterol levels (41%) and increased levels of apolipoprotein A-I (17%) and A-II (17%). Aleglitazar also improved insulin sensitivity by 60% (P = 0.001). Mean body weight was reduced by 5.9% from baseline values with aleglitazar at this dose (P = 0.043).

CONCLUSIONS

Aleglitazar, a dual PPARα/γ agonist, has beneficial effects on both lipid and glucose parameters and may have a therapeutic role in modifying cardiovascular risk factors and improving glycemic control in patients with T2DM.

Peroxisome proliferator-activated receptors, metabolic syndrome and cardiovascular disease

Metabolic syndrome (MetS) is a constellation of risk factors including insulin resistance, central obesity, dyslipidemia and hypertension that markedly increase the risk of Type 2 diabetes (T2DM) and cardiovascular disease (CVD). The peroxisome proliferators-activated receptor (PPAR) isotypes, PPARα, PPARδ/ß and PPARγ are ligand-activated nuclear transcription factors, which modulate the expression of an array of genes that play a central role in regulating glucose, lipid and cholesterol metabolism, where imbalance can lead to obesity, T2DM and CVD. They are also drug targets, and currently, PPARα (fibrates) and PPARγ (thiazolodinediones) agonists are in clinical use for treating dyslipidemia and T2DM, respectively. These metabolic characteristics of the PPARs, coupled with their involvement in metabolic diseases, mean extensive efforts are underway worldwide to develop new and efficacious PPAR-based therapies for the treatment of additional maladies associated with the MetS. This article presents an overview of the functional characteristics of three PPAR isotypes, discusses recent advances in our understanding of the diverse biological actions of PPARs, particularly in the vascular system, and summarizes the developmental status of new single, dual, pan (multiple) and partial PPAR agonists for the clinical management of key components of MetS, T2DM and CVD. It also summarizes the clinical outcomes from various clinical trials aimed at evaluating the atheroprotective actions of currently used fibrates and thiazolodinediones.

Strategies for the development of new PPAR agonists in diabetes

Diabetes is associated with a range of metabolic abnormalities including insulin resistance, atherogenic dyslipidemia, hypertension, and inflammation. These factors are likely to underlie the adverse cardiovascular outcome typically observed in diabetic cohorts. The family of peroxisome proliferator-activated receptors (PPARs) have been implicated in the regulation of a number of physiologic and metabolic pathways. Pharmacologic agonists directed against various PPARs have been shown to have a beneficial impact on these metabolic risk factors. The potential impact on cardiovascular risk with the use of PPAR agonists will be reviewed.

The Role of Peroxisome Proliferator-Activated Receptor beta/delta on the Inflammatory Basis of Metabolic Disease

The pathophysiology underlying several metabolic diseases, such as obesity, type 2 diabetes mellitus, and atherosclerosis, involves a state of chronic low-level inflammation. Evidence is now emerging that the nuclear receptor Peroxisome Proliferator-Activated Receptor (PPAR)β/δ ameliorates these pathologies partly through its anti-inflammatory effects. PPARβ/δ activation prevents the production of inflammatory cytokines by adipocytes, and it is involved in the acquisition of the anti-inflammatory phenotype of macrophages infiltrated in adipose tissue. Furthermore, PPARβ/δ ligands prevent fatty acid-induced inflammation in skeletal muscle cells, avoid the development of cardiac hypertrophy, and suppress macrophage-derived inflammation in atherosclerosis. These data are promising and suggest that PPARβ/δ ligands may become a therapeutic option for preventing the inflammatory basis of metabolic diseases.

A selective peroxisome proliferator-activated receptor alpha agonist, CP-900691, improves plasma lipids, lipoproteins, and glycemic control in diabetic monkeys

Peroxisome proliferator-activated receptors (PPARs) are involved in the regulation of lipid and glucose metabolism. PPARgamma agonists improve insulin sensitivity and hyperglycemia and are effective in treating type 2 diabetes mellitus (T2DM), whereas PPARalpha agonists are used to treat dyslipidemia and atherosclerosis. The goal here was to examine the efficacy of a selective PPARalpha agonist {(S)-3-[3-(1-carboxy-1-methyl-ethoxy)-phenyl]-piperidine-1-carboxylic acid 4-trifluoromethyl-benzyl ester; CP-900691} on lipid, glycemic, and inflammation indices in 14 cynomolgus monkeys with spontaneous T2DM maintained on daily insulin therapy. Monkeys were dosed orally with either vehicle (n = 7) or CP-900691 (3 mg/kg, n = 7) daily for 6 weeks. CP-900691 treatment increased plasma high-density lipoprotein cholesterol (HDLC) (33 +/- 3 to 60 +/- 4 mg/dL, p < 0.001) and apolipoprotein A1 (96 +/- 5 to 157 +/- 5 mg/dL, p < 0.001), reduced plasma triglycerides (547 +/- 102 to 356 +/- 90 mg/dL, p < 0.01), and apolipoprotein B (62 +/- 3 to 45 +/- 3 mg/dL, p < 0.01), improved the lipoprotein index (HDL to non-HDLC ratio; 0.28 +/- 0.06 to 0.79 +/- 0.16, p < 0.001), decreased body weight (p < 0.01) and C-reactive protein (CRP) (1700 +/- 382 to 304 +/- 102 ng/ml, p < 0.01), and increased adiponectin (1697 +/- 542 to 4242 +/- 1070 ng/ml, p < 0.001) compared with baseline. CP-900691 treatment reduced exogenous insulin requirements by approximately 25% (p < 0.04) while lowering plasma fructosamine from 2.87 +/- 0.09 to 2.22 +/- 0.17 mM (p < 0.05), indicative of improved glycemic control. There were no changes in any of the aforementioned parameters in the vehicle group. Because low HDLC and high triglycerides are well established risk factors for cardiovascular disease, the marked improvements in these parameters, and in glycemic control, body weight, and CRP, suggest that CP-900691 may be of benefit in diabetic and obese or hyperlipidemic populations.

Fibrates, glitazones, and peroxisome proliferator-activated receptors

Several decades ago, fibrates were approved for the treatment of dyslipidemia, whereas thiazolidinediones were screened in animal models to improve glucose homeostasis and were subsequently developed for the treatment of type 2 diabetes mellitus. Relatively recently, these drugs were found to act via peroxisome proliferator-activated receptors, nuclear receptors that control lipid metabolism and glucose homeostasis. In this historical perspective, we discuss the history of discovery of the peroxisome proliferator-activated receptors, from the clinical development of their agonists to the subsequent discovery of these receptors and their mechanisms of action, to finally evoke possibilities of targeted pharmacology for future development of selective peroxisome proliferator-activated receptor modulators.

PPARdelta ligand L-165041 ameliorates Western diet-induced hepatic lipid accumulation and inflammation in LDLR-/- mice

Although peroxisome proliferator-activated receptor delta (PPARdelta) has been implicated in energy metabolism and lipid oxidation process, detailed roles of PPARdelta in lipid homeostasis under pathologic conditions still remain controversial. Thus, we investigated the effect of PPARdelta ligand L-165041 on Western diet-induced fatty liver using low-density lipoprotein receptor-deficient (LDLR(-/-)) mice. LDLR(-/-) mice received either L-165041 (5mg/kg/day) or vehicle (0.1N NaOH) with Western diet for 16 weeks. According to our data, L-165041 drastically reduced lipid accumulation in the liver, decreasing total hepatic cholesterol and triglyceride content compared to the vehicle group. Gene expression analysis demonstrated that L-165041 lowered hepatic expression of PPARgamma, apolipoprotein B, interleukin 1 beta (IL-1beta), and interleukin-6. In contrast, L-165041 increased hepatic expressions of PPARdelta, lipoprotein lipase (LPL), and ATP-binding cassette transporter G1 (ABCG1). Our data suggest that L-165041 might be effective in preventing Western diet-induced hepatic steatosis by regulating genes involved in lipid metabolism and the inflammatory response.

PPARs and the cardiovascular system

Peroxisome proliferator-activated receptors (PPARs) belong to the nuclear hormone-receptor superfamily. Originally cloned in 1990, PPARs were found to be mediators of pharmacologic agents that induce hepatocyte peroxisome proliferation. PPARs also are expressed in cells of the cardiovascular system. PPAR gamma appears to be highly expressed during atherosclerotic lesion formation, suggesting that increased PPAR gamma expression may be a vascular compensatory response. Also, ligand-activated PPAR gamma decreases the inflammatory response in cardiovascular cells, particularly in endothelial cells. PPAR alpha, similar to PPAR gamma, also has pleiotropic effects in the cardiovascular system, including antiinflammatory and antiatherosclerotic properties. PPAR alpha activation inhibits vascular smooth muscle proinflammatory responses, attenuating the development of atherosclerosis. However, PPAR delta overexpression may lead to elevated macrophage inflammation and atherosclerosis. Conversely, PPAR delta ligands are shown to attenuate the pathogenesis of atherosclerosis by improving endothelial cell proliferation and survival while decreasing endothelial cell inflammation and vascular smooth muscle cell proliferation. Furthermore, the administration of PPAR ligands in the form of TZDs and fibrates has been disappointing in terms of markedly reducing cardiovascular events in the clinical setting. Therefore, a better understanding of PPAR-dependent and -independent signaling will provide the foundation for future research on the role of PPARs in human cardiovascular biology.

PPARδ activity in cardiovascular diseases: A potential pharmacological target

Activation of peroxisome proliferator-activated receptors (PPARs), and particularly of PPARα and PPARγ, using selective agonists, is currently used in the treatment of metabolic diseases such as hypertriglyceridemia and type 2 diabetes mellitus. PPARα and PPARγ anti-inflammatory, antiproliferative and antiangiogenic properties in cardiovascular cells were extensively clarified in a variety of in vitro and in vivo models. In contrast, the role of PPARδ in cardiovascular system is poorly understood. Prostacyclin, the predominant prostanoid released by vascular cells, is a putative endogenous agonist for PPARδ, but only recently PPARδ selective synthetic agonists were found, improving studies about the physiological and pathophysiological roles of PPARδ activation. Recent reports suggest that the PPARδ activation may play a pivotal role to regulate inflammation, apoptosis, and cell proliferation, suggesting that this transcriptional factor could become an interesting pharmacological target to regulate cardiovascular cell apoptosis, proliferation, inflammation, and metabolism.

Peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) ligands inhibit growth of UACC903 and MCF7 human cancer cell lines

The development of peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) ligands for the treatment of diseases including metabolic syndrome, diabetes and obesity has been hampered due to contradictory findings on their potential safety. For example, while some reports show that ligand activation of PPARbeta/delta promotes the induction of terminal differentiation and inhibition of cell growth, other reports suggest that PPARbeta/delta ligands potentiate tumorigenesis by increasing cell proliferation. Some of the contradictory findings could be due in part to differences in the ligand examined, the presence or absence of serum in cell cultures, differences in cell lines or differences in the method used to quantify cell growth. For these reasons, this study examined the effect of ligand activation of PPARbeta/delta on cell growth of two human cancer cell lines, MCF7 (breast cancer) and UACC903 (melanoma) in the presence or absence of serum using two highly specific PPARbeta/delta ligands, GW0742 or GW501516. Culturing cells in the presence of either GW0742 or GW501516 caused upregulation of the known PPARbeta/delta target gene angiopoietin-like protein 4 (ANGPTL4). Inhibition of cell growth was observed in both cell lines cultured in the presence of either GW0742 or GW501516, and the presence or absence of serum had little influence on this inhibition. Results from the present studies demonstrate that ligand activation of PPARbeta/delta inhibits the growth of both MCF7 and UACC903 cell lines and provide further evidence that PPARbeta/delta ligands are not mitogenic in human cancer cell lines.

Lipids, lipoproteins, and peroxisome proliferator activated receptor-delta

Peroxisome proliferator activated receptors (PPARs) are nuclear receptors activated by small, lipophilic compounds. Typically resident on nuclear DNA, full activation requires heterodimer formation with retinoid X receptor and ligand binding, leading to modulation in the expression of hundreds of genes. Of the 3 described forms, (PPAR-alpha, PPAR-gamma, and PPAR-delta), PPAR-delta has been the least investigated. Preclinical in vitro data show that activation of PPAR-delta, like PPAR-alpha, results in enhancement of fatty acid oxidation, leading to increased energy production in the form of adenosine triphosphate and of energy uncoupling. Microarray data in preclinical models suggest substantial PPAR-delta expression in skeletal muscle. Exercise, which induces upregulation of PPAR-delta in muscle tissue, leads to an increased requirement for an external or serum derived triacylglycerol energy source. This suggests that upregulation of skeletal muscle PPAR-delta would influence lipoprotein composition, this being the major source of combustible substrate. In the first human study using a PPAR-delta agonist, experimental data obtained with GW 501516 (a highly specific PPAR-delta agonist) suggested that upregulated enzymes critical to fatty acid oxidation in human cells enhanced fatty acid and beta-oxidation in skeletal muscle.

PPAR agonists: multimodal drugs for the treatment of type-2 diabetes

Patients with type-2 diabetes mellitus (T2DM) are considered to be at particularly high risk for cardiovascular disease. Over the last decade, the members of the peroxisome proliferator-activated receptor (PPAR) subfamily of nuclear receptors have emerged as valuable pharmacological targets whose activation can normalize metabolic dysfunctions and reduce some cardiovascular risk factors associated with T2DM. PPARalpha agonists, such as the fibrates, can correct dyslipidemia. PPARgamma agonists, such as the thiazolidinediones, act as insulin sensitizers and improve insulin resistance in patients with T2DM. Because of restricted potency and certain side-effects of PPAR agonists, as well as the increasingly epidemic incidence of T2DM, there is a real need for the development of selective PPAR agonists with improved clinical efficacy. This chapter focuses on the PPAR agonists currently used in the clinic, as well as on the discovery and development of the next generation of PPAR agonists.

Identification and characterization of a selective peroxisome proliferator-activated receptor beta/delta (NR1C2) antagonist

The identification of small molecule ligands for the peroxisome proliferator-activated receptors (PPARs) has been instrumental in elucidating their biological roles. In particular, agonists have been the focus of much of the research in the field with relatively few antagonists being described and all of those being selective for PPARalpha or PPARgamma. The comparison of these agonist and antagonist ligands in cellular and animal systems has often led to surprising results and new insights into the biology of the PPARs. The PPARbeta/delta receptor is emerging as an important regulator of energy metabolism, inflammation, and cell growth and differentiation; however, only agonist ligands have been described for this receptor thus far. Here we describe the first report of a PPARbeta/delta small molecule antagonist ligand. This antagonist ligand will be a useful tool for elucidating the biological roles of PPARbeta/delta.

A novel peroxisome proliferator--activated receptor alpha/gamma dual agonist ameliorates dyslipidemia and insulin resistance in prediabetic rhesus monkeys

TAK-559, a newly developed non-thiazolidinedione, activates both peroxisome proliferator-activated receptors alpha and gamma. We investigated the effects of TAK-559 on dyslipidemia and insulin resistance in nonhuman primates. Five adult male obese prediabetic rhesus monkeys were studied on vehicle and after TAK-559 treatment (0.3, 1.0, 3.0 mg/kg per day) for a total of 12 weeks. No significant changes were observed in body weight and fasting plasma glucose, total plasma cholesterol, very low-density lipoprotein-triglyceride, and low-density lipoprotein cholesterol levels. TAK-559 treatment resulted in significant elevation of circulating high-density lipoprotein (HDL) cholesterol levels, consisting of an increase in large HDL particles and a decrease in small dense HDL particles. Nuclear magnetic resonance data exhibited a less atherogenic lipoprotein profile with treatment. Plasma triglyceride and apolipoprotein B-100 levels decreased, whereas apolipoprotein A-I increased during TAK-559 treatment. Hyperinsulinemia and insulin resistance (quantitative insulin sensitivity check index and homeostasis model assessment) were significantly corrected with the highest dose of 3.0 mg/kg per day in these prediabetic monkeys. In addition, no adverse effects on representative liver function parameters were observed during the study period. These results suggest that TAK-559 had beneficial effects on lipoprotein profiles and insulin sensitivity, without any side effect on body weight, which suggests that TAK-559 may provide a potentially safe approach for delaying the onset of type 2 diabetes mellitus and may reduce the risk of cardiovascular disease. The positive effects of TAK-559 in nonhuman primates have led to further clinical trials of TAK-559 in Europe and the United States.

Peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) ligands do not potentiate growth of human cancer cell lines

Ligands for peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) increase skeletal muscle fatty acid catabolism, improve insulin sensitivity, increase serum high-density lipoprotein cholesterol, elicit anti-inflammatory activity and induce terminal differentiation. Contradictory findings are also reported suggesting that PPARbeta/delta ligands potentiate tumorigenesis by increasing cell proliferation, by inhibiting apoptosis through phosphorylation of Akt and by increasing cyclooxygenase-2 (COX2) and vascular endothelial growth factor (VEGF) expression. The contradictory findings could be due to differences in the model system (cancer cell line versus in vivo), differences in cell culture conditions (with and without serum) or differences in ligands. The present study examined the effect of two different PPARbeta/delta ligands (GW0742 and GW501516) in human cancer cell lines (HT29, HCT116, LS-174T, HepG2 and HuH7) cultured in the presence or absence of serum and compared in vitro analysis with in vivo analysis. Neither PPARbeta/delta ligand increased cell growth or phosphorylation of Akt and no increase in the expression of VEGF or COX2 were detected in any cancer cell line in the presence or absence of serum. Similarly, liver, colon and colon polyps from mice administered these PPARbeta/delta ligands in vivo did not exhibit changes in these markers. Results from these studies demonstrate that serum withdrawal and/or differences in ligands do not underlie the disparity in responses reported in the literature. The quantitative nature of the present findings are inconsistent with the hypothesis that cancer cell lines respond differentially as compared with normal cells, and provide further evidence that PPARbeta/delta ligands do not potentiate tumorigenesis.

Peroxisome proliferator-activated receptor-delta polymorphisms are associated with physical performance and plasma lipids: the HERITAGE Family Study

We tested the hypothesis that peroxisome proliferator-activated receptor-delta (PPARdelta) gene polymorphisms are associated with cardiorespiratory fitness and plasma lipid responses to endurance training. Associations between the PPARdelta exon 4 +15 C/T and exon 7 +65 A/G polymorphisms and maximal exercise capacity and plasma lipid responses to 20 wk of endurance training were investigated in healthy white (n = 477) and black (n = 264) subjects. In black subjects, the exon 4 +15 C/C homozygotes showed a smaller training-induced increase in maximal oxygen consumption (P = 0.028) than the C/T and T/T genotypes. Similarly, a lower training response in maximal power output was observed in the exon 4 +15 C/C homozygotes (P = 0.005) compared with the heterozygotes and the T/T homozygotes in black subjects, and a similar trend was evident in white subjects (P = 0.087). In white subjects, baseline apolipoprotein A-1 (Apo A-1)levels were higher in the exon 4 +15 C/C (P = 0.011) and exon 7 +65 G/G (P = 0.05) genotypes compared with those in the other genotypes. In white subjects, exon 4 +15 C/C (P = 0.0025) and exon 7 +65 G/G (P = 0.011) genotypes showed significantly greater increases in plasma high-density lipoprotein-cholesterol (HDL-C) levels with endurance training than in the other genotypes, whereas in black subjects the exon 4 +15 CC homozygotes tended to increase (P = 0.057) their Apo A-1 levels more than the T allele carriers. DNA sequence variation in the PPARdelta locus is a potential modifier of changes in cardiorespiratory fitness and plasma HDL-C in healthy individuals in response to regular exercise.

Ligand activation of peroxisome proliferator-activated receptor-beta/delta(PPARbeta/delta) inhibits cell growth of human N/TERT-1 keratinocytes

The functional role of peroxisome proliferator-activated receptor-beta(PPARbeta; also referred to as PPARdelta) in epidermal cell growth remains controversial. Recent evidence suggests that ligand activation of PPARbeta/delta increases cell growth and inhibits apoptosis in epidermal cells. In contrast, other reports suggest that ligand activation of PPARbeta/delta leads to the induction of terminal differentiation and inhibition of cell growth. In the present study, the effect of the highly specific PPARbeta/delta ligand GW0742 on cell growth was examined using a human keratinocyte cell line (N/TERT-1) and mouse primary keratinocytes. Ligand activation of PPARbeta/delta with GW0742 prevented cell cycle progression from G1 to S phase and attenuated cell proliferation in N/TERT-1 cells. Despite specifically activating PPARbeta/delta as revealed by target gene induction, no changes in PTEN, PDK and ILK expression or downstream phosphorylation of Akt were found in either N/TERT-1 cells or primary keratinocytes. Further, altered cell growth resulting from serum withdrawal and the induction of caspase-3 activity by ultraviolet radiation were unchanged in the absence of PPARbeta/delta expression and/or the presence of GW0742. While no changes in the expression of mRNAs encoding cell cycle control proteins were found in response to GW0742, a significant decrease in the level of ERK phosphorylation was observed. Results from these studies demonstrate that ligand activation of PPARbeta/delta does not lead to an anti-apoptotic effect in either human or mouse keratinocytes, but rather, leads to inhibition of cell growth likely through the induction of terminal differentiation.

PPARδ agonists and metabolic diseases

Peroxisome proliferator-activated receptors (PPARs) are the key transcription factors regulating lipid metabolism and energy homeostasis. PPARalpha and PPARgamma are known therapeutic targets for hypertriglyceridemia and type 2 diabetes, respectively. The physiologic function of the third member, PPARdelta, has been difficult to define due to its broad tissue distribution. Through the creation of transgenic mouse models and identification of high-affinity synthetic ligands, the diverse activities of PPARdelta in several metabolically active tissues, including skeletal muscle, adipose tissue, liver, and macrophages, have recently been revealed. These metabolic activities of PPARdelta implicate the potential use of PPARdelta agonists to treat metabolic diseases, including atherosclerosis and insulin resistance.

Peroxisome proliferator-activated receptor-delta, a regulator of oxidative capacity, fuel switching and cholesterol transport

Synthetic agonists of peroxisome proliferator-activated receptor (PPAR)-delta have shown a promising pharmacological profile in preclinical models of metabolic and cardiovascular disease. At present, the pharmaceutical development of these drugs exploits the potential to raise plasma HDL-cholesterol in animals and their insulin-sensitising and glucose-lowering properties. PPAR-delta agonists have also proven to be powerful research tools that have provided insights into the role of fatty acid metabolism in human physiology and disease. Activation of PPAR-delta induces the expression of genes important for cellular fatty acid combustion and an associated increase in whole-body lipid dissipation. The predominant target tissue in this regard is skeletal muscle, in which PPAR-delta activation regulates the oxidative capacity of the mitochondrial apparatus, switches fuel preference from glucose to fatty acids, and reduces triacylglycerol storage. These changes counter the characteristic derangements of insulin- resistant skeletal muscle but resemble the metabolic adaptation to regular physical exercise. Apart from effects on fuel turnover, there is evidence for direct antiatherogenic properties, because PPAR-delta activation increases cholesterol export and represses inflammatory gene expression in macrophages and atherosclerotic lesions. Whereas conclusions about the full potential of PPAR-delta as a drug target await the result of large scale clinical testing, ongoing investigation of this nuclear receptor has greatly improved our knowledge of the physiological regulation of whole-body fuel turnover and the interdependence of mitochondrial function and insulin sensitivity.

Triglyceride:high-density lipoprotein cholesterol effects in healthy subjects administered a peroxisome proliferator activated receptor delta agonist

OBJECTIVE

Exercise increases fatty acid oxidation (FAO), improves serum high density lipoprotein cholesterol (HDLc) and triglycerides (TG), and upregulates skeletal muscle peroxisome proliferator activated receptor (PPAR)delta expression. In parallel, PPARdelta agonist-upregulated FAO would induce fatty-acid uptake (via peripheral lipolysis), and influence HDLc and TG-rich lipoprotein particle metabolism, as suggested in preclinical models.

METHODS AND RESULTS

Healthy volunteers were allocated placebo (n=6) or PPARdelta agonist (GW501516) at 2.5 mg (n=9) or 10 mg (n=9), orally, once-daily for 2 weeks while hospitalized and sedentary. Standard lipid/lipoproteins were measured and in vivo fat feeding studies were conducted. Human skeletal muscle cells were treated with GW501516 in vitro and evaluated for lipid-related gene expression and FAO. Serum TG trended downwards (P=0.08, 10 mg), whereas TG clearance post fat-feeding improved with drug (P=0.02). HDLc was enhanced in both treatment groups (2.5 mg P=0.004, 10 mg P<0.001) when compared with the decrease in the placebo group (-11.5+/-1.6%, P=0.002). These findings complimented in vitro cell culture results whereby GW501516 induced FAO and upregulated CPT1 and CD36 expression, in addition to a 2-fold increase in ABCA1 (P=0.002). However, LpL expression remained unchanged.

CONCLUSIONS

This is the first report of a PPARdelta agonist administered to man. In this small study, GW501516 significantly influenced HDLc and TGs in healthy volunteers. Enhanced in vivo serum fat clearance, and the first demonstrated in vitro upregulation in human skeletal muscle fat utilization and ABCA1 expression, suggests peripheral fat utilization and lipidation as potential mechanisms toward these HDL:TG effects.